Compressed natural gas (CNG) fuel systems require different maintenance than conventional fuel systems. Technicians should regularly inspect and replace the fuel filter, which removes any oil or contaminants in the fuel. This filter generally needs to be replaced annually by a qualified service facility; owners should check their owner's manual for the specific requirements of their vehicle. In some cases, filters should be drained of any contaminants on a more regular basis, depending upon the application.
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CNG fuel tanks have a useful life of 15 to 20 years, depending on their construction and how they were certified by the original manufacturer. Because there is no way to safely "requalify" tanks for extended use, once a tank reaches its expiration date, it must be replaced. All CNG tanks carry a label that indicates "DO NOT USE AFTER (EXPIRATION DATE)." For instance, it might read "DO NOT USE AFTER 01/2027." The CNG tank expiration date may also be found on a label near the fueling connector or on a label under the hood. CNG vehicle owners should make a habit of noting the expiration date at routine service intervals. They should also be aware that there is no national system to notify owners when their tanks reach their expiration date; it is the vehicle owner's or fleet manager's responsibility to have the tank replaced at the end of its life. For more information on CNG tank end-of-life issues, see the Compressed Natural Gas Fuel Tank Defueling, Decommissioning, and Disposal video.
CNG tanks should be replaced by a qualified service facility. This type of service facility will include properly trained personnel and equipment to safely vent the CNG from the tank and purge the tank with nitrogen to eliminate any pressure or fire danger associated with residual CNG in the tank. Once the tank is safely purged of any natural gas, the expired CNG tank must be rendered unusable and discarded. See the Compressed Natural Gas Fuel Tank Defueling, Decommissioning, and Disposal video or CSA EXP 2.1, Defueling, decommissioning, and disposal of compressed natural gas vehicle fuel containers best practices document for more information.
Performing a regular safety inspection of the CNG storage tanks is a critical maintenance requirement for CNG fuel systems. Damage from road debris can threaten the integrity of CNG tanks. Exposure to certain chemicals may cause corrosion and cracking as well. Because these tanks are pressurized to 3,600 pounds per square inch (psi), what appears to be insignificant damage could pose significant danger due to tank quality degradation. For these reasons, tanks on vehicles with a gross vehicle weight rating greater than 10,000 lbs should be inspected in a qualified service facility once every year (12 months). Light-duty vehicle tanks should be inspected every three years (36 months) or every 36,000 miles, whichever comes first (U.S. DOT National Highway Traffic Safety Administration FMVSS 304). In addition, tanks must be inspected after any fire, accident, or other incident that could cause damage to the tank.
Tanks that are mounted in the underbody of a vehicle are at higher risk of impact by debris on the road or becoming damaged from impact, such as driving over a curb. Owners can locate certified inspectors by searching for "Certified CNG Fuel System Inspector" on the CSA Group website or using the Natural Gas Vehicle Institute Certified Inspector Lookup Tool. The CSA Group is the standards-writing body in the United States for natural gas appliances and related equipment.
New CNG vehicles will have labels located on the tank, near the fueling connector, or under the hood in the engine compartment defining the tank inspection and expiration dates. For used vehicles, owners should obtain the most recent inspection record from the previous owner and should have a qualified service facility perform a new inspection before purchasing the vehicle. Likewise, if you are buying one or more used CNG tanks for a converted vehicle, the tank should be verified to have a sufficient useful lifetime left before the defined expiration date (see above) and should be inspected by a certified CNG fuel system inspector.
After the first inspection, vehicle owners or fleet managers should keep a record of the most recent inspection and when the next one is due. Many inspection facilities provide reminder stickers for oil changes that list the date and mileage when the next oil change is due; it is recommended to use these reminder stickers for CNG tank inspections as well. CNG vehicle owners may even have two reminder stickers, one for oil and one for the tank inspection.
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CNG Vessel Sizing
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(Mechanical)
(OP)
19 Feb 16 16:53I am interested in a Compressed Natural Gas (CNG) vessel to supply natural gas to heat a building because the piping from our existing infrastructure is far away and it's going to cost an insane amount in scaffolding, etc. to pipe it, there. All I know is that the building's HVAC units require a total of about 34m³/hr of gas (1200ft³/hr). I would like the vessel sized to last something near 4 weeks.My pathetic Google searching keeps showing wet capacity of vessels and "gas capacities" that don't really mean much without the pressure. I suppose my 34m³/hr also does not mean much without a pressure; but, IIRC, gas is delivered to heaters at barely any pressure at all, isn't it? Something like 1psi? Arbitrarily close to atmospheric?My crack at it would be something along the lines of (24hr/day)(30days/month)(34m³/hr) = 24480 m³ of atmospheric gas per month.Then, convert that gas to a volume of liquid by looking up what pressure natural gas liquifies at. Find a vessel that is 1.25x the wet volume of natural gas (because you can't fill them >80%). Verify that it is rated for a suitable pressure and see what it costs.Profit?Please point out my errors and mock or flame accordingly.Thanks!
(Mechanical)
19 Feb 16 21:40You may want to contact an area distributor that would be willing to do the engineering, installation and supply. For instance, in my region, a chemical plant owner was approached by a LNG distributor to install all piping, associated controls and bulk heads and to deliver LNG by trucking in the product 90 miles from the distributor source. LNG is stored in two tanker trailers connected to the LNG bulk head and delivery is made periodically each week. This offer by the LNG distributor was presented after the plant owner had invested in the installation of a 30,000 gallon propane tank. Try a similar approach.
(Mechanical)
19 Feb 16 22:01First off I'm going to assume that your units are SCM and SCF. Your first step is OK-ish, but then you veer from the track. The 30 days/month term doesn't add any value. Just multiply your volume/day times number of days. I get 806,870 SCF [22,848 SCM]. You will not liquify the natural gas in this process, just raise it to a very high gas pressure.Density of methane at 14.73 psia [101.56 kPaa] and 60F [15.56 C] is 0.0451 lbm/SCF [0.723 kg/SCM] (assuming compressibility is negligible). CNG is stored at a nominal 3600 psia [24.8 MPaa] and 70F [21.1 C] so the density is 12.346 lbm/ft^3 [197.7 kg/m] (calculating compressibility as 0.876). This is all you need. The volume of the vessel (with no safety factor) is:Vol= Q*ρ/ρ=2950 ft[84 mIf you pick a 90-inch [2.3 m] diameter receiver then it would have to be 67 ft [20.3 m] long. Getting a vessel rated for CNG pressures much bigger than 90 inch is outrageously expensive, but then getting a 90-inch pressure vessel 67 ft long would be tens of millions of dollars.The storage gets smaller with LNG, but sources of LNG and the gassification equipment for your load are hard to come by. There is a reason that people pay the cost to run pipe to their buildings.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
(Materials)
20 Feb 16 00:45The cost/difficulty of getting LNG very greatly by location.In some locals LPG (propane, butane) sell for the same cost per BTU as nat gas.I would check with suppliers and see what they can offer.
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P.E. Metallurgy, Plymouth Tube
(Chemical)
20 Feb 16 05:44Due to the volume and pressure of this flammable gas inventory, this unit will have to kept some distance away from this building - check with your safety engineer how far away. It will also need road access for removing spent vessels and installation of a fully pressurised charge. Operational safety problems during removal and reload also. All these taken into account ?
(Petroleum)
20 Feb 16 11:44Dave has the numbers right, but not the cost. CNG systems are now made in their thousands to transport gas in India and Asia.A standard trailer holds about 20m3 volume at 200 to 250 bar. So for a month you need about 4 or 5 trailers.How much? Don't know but they often make these things from fibre glass a multiple small tubes.Search CNG containers.Here's an example. www.weldship.com/about-us/recent-news-and-events1.... Don't confuse LNG, LPG with CNG. LNG is not currently available in that size of system, LPG is, but usually more expensive than gas. CNG is now very common in lots of places so the cost of equipment is low and safety issues have been worked out.Let us know if you're going ahead.
Remember - More details = better answers
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(Mechanical)
20 Feb 16 14:44That configuration makes more sense than trying to do it in one vessel. Probably 6 of the 22" X 40' would do the job, I don't know what these tubes cost, but I would expect them to not be inexpensive.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
(Petroleum)
20 Feb 16 16:02What quite a few places do is have a set of multiple tanks and then get regular deliveries of cng. The first bit just flows across then they plug in a compressor to pull the gas volume down from the trailer into storage. Lots of tanker loads though for the OP sized load, pipelines are a much better idea.....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
(Mechanical)
(OP)
24 Feb 16 20:44zdas04: I should have said LNG from the start. I mentioned calculating the liquid volume in the post because I was pretty sure that a CNG vessel would be WAY too big; but, I put in a misleading thread title.
I think the idea is dead before it's off the ground. We are unfortunately stuck with Natural Gas which is infuriating me that those selections are set in stone when propane should work just as well and is super prevalent.
I pictured the LNG equivalent of a rental propane bullet that a company like Superior could supply and fill in required intervals. The filling is what kills it as I don't think there are any LNG trucks that can supply us in our region.
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